1. Field of the Invention
The present invention relates to a white light emitting device, and more particularly, to a white light emitting device using a light emitting diode (LED) and having high color reproducibility and high reliability.
2. Description of the Related Art
As a next generation light source product for replacing a conventional light source such as a fluorescent lighting or a backlight source of a liquid crystal display (LCD) display, light emitting diodes (LEDs) with low power consumption and excellent light efficiency receive attentions. To variously apply LEDs with high efficiency, there have been performed a lot of researches on white light emitting devices using an LED. As a method of embodying a white light using LEDs, there is generally used a method of coating blue LED with a yellow phosphor.
However, in the case of white light emitting devices using a blue LED and a yellow phosphor, since there is no definite distinction between green and red wavelength areas and a light intensity in a long wavelength area is very low, color reproducibility is not high enough. Due to this, white light emitting devices using a blue LED and a yellow phosphor are not suitable for products requiring high color reproducibility, such as a color screen of a mobile phone and an LCD display for a computer monitor.
FIG. 1A is a diagram illustrating a light emitting spectrum of a conventional white light emitting device using an LED. The light emitting spectrum relates to a white light outputted from a white light emitting device including a blue LED and an yttrium aluminum garnet (YAG)-based yellow phosphor excited by the blue LED. As shown in FIG. 1A, in the spectrum, not only there is no definite distinction but also there is shown a relatively low light intensity in a long wavelength area such as a red wavelength area. Such light emitting spectrum characteristics are disadvantageous in an aspect of color reproduction.
FIG. 1B is a diagram illustrating spectrums obtained when transmitting a white light having the spectrum of FIG. 1 to blue, green, and red color filters. As shown in FIG. 1B, though separating the white light into blue, green, and red spectrums by using the color filters, there is a considerable overlap portion between a green light spectrum and a red light spectrum. In addition, the red light spectrum separated by the red color filter has a considerably low relative intensity in a wavelength of 600 nm or more. Accordingly, when using the white light having the spectrum of FIG. 1A as a backlight of an LCD display, the LCD display shows color reproducibility not enough. Actually, when using a mixture of a blue InGaN-based LED and a YAG-based phosphor as a light source for a backlight, there is embodied just color reproducibility of about 60 to 65%, comparing with national television system committee (NTSC). It is difficult to express various colors by using the color reproducibility of such degrees.
In addition to the described mixture of a blue LED and a yellow phosphor, there is provided a white light emitting device using a mixture of a blue LED and red and green phosphors. When using the red and green phosphors as described above, color reproducibility thereof is increased to some degrees. However, this is not enough. Also, due to instability of phosphor materials, such as a damage caused by external energy, red or green phosphors generally used for white light emitting devices are unreliable.
Particularly, sulfide phosphors having a relatively high brightness and a wide color distribution has excellent optical properties. However, there is a problem in material stability, where a structure there is easily destroyed. For example, a red sulfide phosphor such as SrS:Eu may react to a platinum catalyst added in a silicone resin or an epoxy resin for encapsulation. Also, when exposed to humidity, there may be generated a corrosive gas having a bad effect on products. A red oxide phosphor such as Gd2O3:Eu may be considered instead of the red sulfide phosphor. However, such red oxide phosphor has a low brightness and a narrow color distribution. Accordingly, a white light emitting device using the red oxide phosphor is incapable of embodying a desirable brightness and color reproducibility.